In a work environment, the accumulated amount of noise, or dose in terms of an average noise level, and the maximum level of noise to which an individual has been exposed during a workday are important to occupational safety and to the health of the individual. Industry and governmental agencies in countries throughout the world, such as the Occupational Safety and Health Administration (OSHA) in the United States, require accurate noise data measurements.
Examples of such noise data measurements include impulse noise, continuous noise, and an eight-hour time-weighted average (“TWA”) that is also referred to as “daily personal noise exposure”. Impulse noise relates to noise of very short duration. Continuous noise relates to noise that is longer in duration than impact noise, extending longer than 500 milliseconds. Eight-hour TWA relates to the average of all levels of impulse and continuous noise to which an employee is exposed during an eight-hour workday. The OSHA maximum level for impulse noise is 140 dBSPL measured with a fast peak-hold sound level meter (“dBSPL” stands for sound pressure level, or a magnitude of pressure disturbance in air, measured in decibels, a logarithmic scale). The maximum level for continuous noise is 115 dB(A) (read on the slow average “A” scale). OSHA regulations limit an eight-hour TWA to 90 dB(A). If employees are exposed to eight-hour TWAs between 85 and 90 dB(A), OSHA requires employers to initiate a hearing conservation program which includes annual hearing tests.
Among the many noise sources that are encountered in the workplace, measuring the exposure to noise related to telephone headsets is especially problematic. Standard noise exposure measurement procedures described in the United States Code of Federal Regulations at 29 CFR 1910.95 and International Organization for Standardization (ISO) 1999 can only be performed for open-field environmental noise that can be measured with a sound level meter. An “open-field” environment is an environment where the sound or noise sources are at a distance from a person's ear. The sound or noise environment can be a single or a combination of many acoustic fields, i.e. free field, partially reflected field, diffuse field and reverberant field. Noise exposure from headsets is different from the “open-field”. The sound is localized at or inside of the users' ear. It is necessary to transfer the measured earphone or headset sound pressure levels to the “open-field” before comparing them to the regulatory TWA noise exposure limits.
In the prior art, the most accurate noise exposure measurement technique for headsets, and other equipment that uses sound-sources placed close to the ear, is the MIRE (Microphone in Real Ear) technique as defined in ISO 11904-01. However fitting a calibrated microphone in the ear is uncomfortable and obtrusive, and the measurement equipment is bulky and needs a skilled operator. Therefore the MIRE technique is unsuitable for measuring the noise exposure of a mobile user, without seriously affecting his mobility and normal working practices.
Another prior art method for noise exposure measurement in a corded headset utilizes a buffered splitter in the headset cable to drive a second headset with the same signal as the user's headset. The second headset is placed on a head and torso simulator (HATS) in order to make calibrated sound level measurements. This method avoids the difficulty and discomfort of fitting an in-ear microphone. However, it is unsuitable for a mobile user wearing a wireless headset because a wireless headset has no headset cable in which to insert a splitter, and the measurement equipment (HATS and sound level meter/dosimeter) is not portable, so it can only be used to measure noise exposure for static users such as call-centre agents. Measurements cannot be made for mobile headset users without significantly altering their headset usage patterns. Furthermore, this method is not suitable for headsets that have built-in, user adjustable, receive-volume controls, as it is difficult to ensure that the volume controls for both the user's headset and the test headset are set identically.
Another prior art method for measuring noise exposure in corded headsets is based on the alternative daily noise exposure measurement method described in ITU Telecommunication Standardization Sector (ITU-T) Recommendation P.360 and European Telecommunications Standardization Institute (ETSI) EG 202 518. This alternative noise exposure measurement method does not require the expensive and bulky test equipment that the standard HATS measurement technique uses at the test site. Its principle of operation is (1) electrical monitoring of the signal at the input of the headset's speaker (i.e., after all volume controls), (2) estimation of the acoustic pressure at the eardrum by means of a statistically validated model of the headset response, as characterized on the HATS in the laboratory, and (3) calculation of the equivalent sound pressure level of the received speech in the diffuse field, according to ISO 11904.
The accuracy of this alternative measurement method depends primarily on the accuracy of the model used for acoustic pressure estimation at the eardrum. Using a generic model of the speaker/ear transfer function for a particular headset type typically results in daily personal noise exposure measurements within a few decibels (dB) of the “true” value. The measurement error is caused by variations between the sensitivity or frequency response of the generic model and the specific headset that is in use. These variations exist due to normal manufacturing tolerances. However if the model of the headset response is based on an individual HATS measurement for the specific headset, then the alternative measurement method is as accurate as the standard HATS headset noise exposure measurement method. In fact it may be slightly more accurate, because the measurement uses the characteristics of the actual headset that is in use, rather than using a splitter cable and a second headset that is similar to it, but not identical. U.S. Pat. No. 6,826,515 entitled “Headset Noise Exposure Dosimeter” and assigned to Plantronics, Inc., describes a noise dosimeter integrated with a corded headset.
In most countries outside the United States, the standard daily personal noise exposure calculation method, defined in ISO 1999, starts by calculating the equivalent continuous A-weighted sound pressure level LAeq,Te. The calculation continuously integrates sound pressure level with respect to time, divides by the duration of the working day, and then scales the result to account for differences between the actual time worked and the 8-hour “reference” duration, as shown in equations (1) and (2).
                              L                      Acq            ,            Te                          =                  10          ⁢                                          ⁢                      log            10                    ⁢                                                                1                                                      t                    2                                    -                                      t                    1                                                              ⁢                                                ∫                                      t                    1                                                        t                    2                                                  ⁢                                                                                                    (                                                                              p                            A                                                    ⁡                                                      (                            t                            )                                                                          )                                            2                                                              p                      0                      2                                                        ⁢                                      ⅆ                    t                                                                                                                      (        1        )                                          L                      EX            ,                          8              ⁢              h                                      =                              L                          Aeq              ,              Te                                +                      10            ⁢                                                  ⁢                          log              10                        ⁢                                                                          T                  e                                                  T                  0                                                                                                      (        2        )                            where:                    LAeq,Te=equivalent continuous A-weighted sound pressure level            pA(t)=A-weighted sound pressure level, in Pascals, as a function of time            p0=reference pressure (=20 μPa)            LEX,8h=Daily personal noise exposure            Te=effective duration of the working day (normally Te=t2−t1)            T0=reference duration (=28800 seconds, equivalent to 8 hours)                        
In the United States, a different measurement method is used set forth in 29 CFR 1910.95, in which only sound levels above 80 dB(A) contribute to daily personal noise exposure. Also, the measurement method uses a 5 dB exchange rate instead of a 3 dB exchange rate, where doubling the duration of noise exposure corresponds to a 5 dB increase in daily personal noise exposure instead of a 3 dB increase. The time-weighted averaging process of equations (1) and (2) inherently has a 3 dB exchange rate, so cannot be used for USA daily personal noise exposure measurements. An alternative measurement procedure is used, based on the American National Standards Institute (ANSI) S1.25 noise dose calculations.
Direct calculation of daily personal noise exposure using equations (1) and (2) has a disadvantage: It requires accurate measurement of elapsed time (t2, t1, Te), and thus requires a “real time” clock. In equation (1), t1 and t2 are the times at which the measurement starts and stops, and the expression (t2−t1) is the length of the working day. A “real-time” clock is an electronic hardware or software module from which a microprocessor or DSP can read the time and date. It maintains its time-keeping accuracy regardless of whether the host device in which it is embedded is switched on or off. This is normally achieved by using a battery or super-capacitor to provide back-up power to the real-time clock module when the host device is powered down. Some common examples of real-time clocks are the system clock in a PC and the recording timer in a video recorder.
Wireless headsets do not normally contain a real-time clock module, but they invariably contain a very stable crystal-controlled frequency reference that is used by their microprocessor and DSP. Thus, as long as the headset is switched on, it can accurately measure the duration of events, or schedule actions to occur at precisely defined intervals. However when switched off, all timing information is lost. When the headset is switched on again, the time period for which the headset has been off is unknown. Therefore if a headset is switched off and on again between the times that an event starts and ends, the duration of that event cannot be measured. Adding a discrete real-time clock module to a wireless headset significantly increases both the component cost and the headset size, so is not a preferred option. Thus, the direct calculation of daily personal noise exposure using equations (1) and (2) described above cannot be evaluated on a wireless headset.
As a result, there is a need for improved methods and apparatuses for measuring noise exposure in wireless headsets.